Re: [PATCH v2] 9p/trans_fd: perform read/write with TIF_SIGPENDING set

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2022/09/04 8:39, Dominique Martinet wrote:
> Is there any reason you spent time working on v2, or is that just
> theorical for not messing with userland fd ?

Just theoretical for not messing with userland fd, for programs generated
by fuzzers might use fds passed to the mount() syscall. I imagined that
syzbot again reports this problem when it started playing with fcntl().

For robustness, not messing with userland fd is the better. ;-)

> 
> unless there's any reason I'll try to find time to test v1 and queue it
> for 6.1

OK.

> We seem to check for EAGAIN where kernel_read/write end up being called
> and there's a poll for scheduling so it -should- work, but I assume this
> hasn't been tested much and might take a bit of time to test.

Right. Since the I/O in kernel side is poll based multiplexing, forcing
non-blocking I/O -should- work. (But I can't test e.g. changes in CPU time
usage because I don't have environment to test. I assume that poll based
multiplexing saves us from doing busy looping.)

We are currently checking for ERESTARTSYS and EAGAIN. The former is for
non-socket fds which do not have O_NONBLOCK flag, and the latter is for
socket fds which have O_NONBLOCK flag. If we enforce O_NONBLOCK flag,
the former will become redundant. I think we can remove the former check
after you tested that setting O_NONBLOCK flag on non-socket fds does not break.